First, the rated load of the power transformer is calculated on both sides when the apparent
power and phase-to-phase voltage are known.
GUID-FD7DAEE6-80AB-4C5E-98C7-863063099802 V1 EN (Equation 68)
I
nT
rated load of the power transformer
S
n
rated power of the power transformer
V
n
rated phase-to-phase voltage
Next, the settings for the CT ratio correction can be calculated with the formula:
CT ratio correction
I
I
n
nT
=
GUID-F950DE9C-6AFB-4DF9-B299-6631693C55C0 V1 EN (Equation 69)
I
n
nominal primary current of the CT
After the CT ratio correction, the measured currents and corresponding setting values of
87L are expressed in multiples of the rated power transformer current Ir (×Ir) or
percentage value of Ir (%Ir).
An example shows how the CT ratio correction settings are calculated; when the rated
power of the transformer is 5 MVA, the ratio of CTs on the 20 kV side is 200/1 and that
on the 10.5 kV side is 300/1.
1MAC059074-MB A Section 4
Protection functions
615 series ANSI 467
Technical Manual
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